Ian,

I think a CT with a 1:1000 ratio may work better. A quick look on
Digikey turned up this:
http://rocky.digikey.com/scripts/ProductInfo.dll?Site=3DUS&V=3D196&M=3DAC=
-1050
(datasheet:
http://rocky.digikey.com/WebLib/Amveco-Talema/Web%20Data/AC1050.pdf)
(I'm sure you'll be able to find it locally in Australia too.)

Terminate the output into a 100 ohm resistor and you should have a 0 ~
5V for 0 ~ 50A of current through the CT. Rectify that, put a cap and a
small resistive load on the DC side, and a zener in series with a
resistor to gnd to protect PIC from over voltage and spikes and feed it
to the D/A.=20

A similar approach is used in a UPS for sensing load that I am trying to
fix these days (rather unsuccessfully.)

Hope this helps.

Shahid.

-----Original Message-----
From: Ian McLean [mailto:ianmm@OPTUSNET.COM.AU]=20
Posted At: Thursday, February 19, 2004 5:08 PM
Subject: [EE:] Current Transformers


Hi,

I posted this earlier, but it never appeared on the list - in fact until
Jinx just posted then, nothing seems to have hit the list since about 10
hours ago.  Has PICList been off the air for awhile ?

Anyhow, I'll repost my question now.

I would like to measure AC current using PIC A/D but am not sure what
would be the best way to do it.

I need to measure current at line voltage and frequency (240VAC, 50Hz).
I would like to be able to measure up to 50A current (yes, I know it is
big, 12kW max., but that is what I need), and convert that into a 0-5V
DC range for the PIC A/D, so I can display the current on an LCD
display.  I have no problem with the PIC side, A/D, LCD, etc., but am
not sure how to do the current conversion.  I have done DC current
reading on the PIC before up to 20A, simply by using a sense resistor
and op-amp, so I am half way to knowing what I have to do.

I figured I should probably use a CT (current transformer) in series
with the hot wire of the AC supply, that does a 50A:5A ratio conversion,
but having trouble finding one of these.  Can anyone point me in the
right direction ?  On the secondary side of the CT, I would then 1/2
wave rectify the output and pass the resulting 0-5A DC through a sense
resistor in parallel with the CT secondary winding, take the voltage
drop across the sense resistor, amplify it with a general purpose
op-amp, and feed that to the PIC A/D.  This is the only way I know (or
at least think I know)to do this, and it does seem a rather roundabout
way of reading large AC currents.

Alternatively, how hard would it be to wind my own CT for this purpose,
or can someone suggest a clever alternate solution ?

PS: This is for a personal project.  Accuracy is not paramount.  If I
can get it accurate to within an amp or two, I am happy.

Rgs
Ian.

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